The invention relates to electric circuit assemblies, and more particularly to an improved manufacturing method in combination.
Electric circuit assemblies are known, and typically include an electrically insulating thermally conductive nonmetallic refractory substrate, e.g. ceramic, having electrically conductive metallic lead frames, e.g. copper, mounted on top of the substrate, and semiconductor elements mounted on the lead frames. The structure is covered by an insulating housing which in turn is mounted to a heat sink such that the bottom surface of the ceramic substrate is in intimate contact with the heat sink. Examples of such circuit assemblies are shown in U.S. Patent Nos. 3,958,075, 4,156,148, 4,196,411, 4,215,235, 4,218,724, 4,250,481, 4,266,140, 4,394,530, 4,449,165, 4,449,292, 4,488,202, 4,498,120, 4,546,410, 4,546,411, 4,554,613, 4,574,162, 4,577,387, 4,630,174, and 4,700,273, incorporated herein by reference.
The copper lead frames are mounted to the ceramic substrate in various manners, for example by a solder reflow operation, or by a direct bond operation.
Direct bonding of the copper to the ceramic is known, and involves placing the lead frame in contact with the substrate, providing a reactive gas atmosphere which at an elevated temperature will react with the lead frame to form a eutectic, heating the lead frame and the substrate to a temperature below the melting point of the lead frame in the reactive atmosphere to form the eutectic with the lead frame which wets the lead frame and the substrate, and cooling the lead frame and the substrate with the lead frame bonded to the substrate. This direct bonding process is disclosed in U.S. Pat. Nos. 3,766,634, 3,993,411, 3,994,430, 3,911,553, 3,744,120, 3,854,892, and 4,129,243, incorporated herein by reference.
While the above noted direct bond process provides a good strong bond and eliminates the need for a solder reflow operation, it has been found that in an electric circuit assembly application as above, such process leaves the lead frames too soft. In fact, it has been found that lead frames so processed are subject to collapse upon push-on fastener connection thereto, and hence not suitable for electric circuit assembly application of the type noted above.
The present invention provides a simple yet effective method of making an electric circuit assembly incorporating the above noted direct bond process. The invention includes in combination a simple work hardening step for the lead frame to stiffen same and enable push-on fastener connection to the lead frame without collapse thereof.